Much equipment, such as computers and disc drives, generate sufficient heat to require the use of fans to cool the components. Fans, unfortunately, can generate sufficient acoustic noise so that the work place becomes, at least, an unpleasant place to work.
In response to this, many types of acoustic shielding have been devised. For example, U.S. Pat. No. 4,508,486 shows a ventilation fan having a perforated inner casing, a solid outer casing and a porous, sound absorbing material between the two. U.S. Pat. No. 3,540,547 to Coward shows a complex, 5 layer sound absorbing panel including an inner perforated layer, two sound absorbing layers and two solid layers. U.S. Pat. No. 3,947,148 to Holt shows a fan having a cylindrical duct with slots in the duct. A foam layer surrounds the duct and extends through the slots to minimize the clearance between the ends of the fan blades and the inner wall of the duct. Any sound reduction scheme must, however, ensure that any reduction in sound levels does not create an unacceptable reduction in flow rates, does not have an unacceptable cost and does not reduce the ability of the user to work with the equipment to any substantial extent.